This page contains a blow by blow account of the
trials and tribulations I went through along my journey of building a Vibroclone.

Keep in mind, this was my first "ever" project amp. Up until this
point my knowledge of amps was largely academic. And my experience with working
on amps up until this project, was limited to simple mods and fixes. So don't be
discouraged if you're new at this too. If I can do it, so can you...

Important questions to ask yourself before you start ?

There are a handful of different ways to approach a Vibro-Clone project,
each of which will produce a different end result. So before you start, you
should define what it is you hope to end up with, how much money you're
willing to spend, and how much time you're willing to spend. The broad
definitions of Vibro-Clone possibilities can be summed up as follows:

Note: Costs are "your" price for parts only, and do not
include any labor (since you'll be the one doing the work).

What parts will I need ?

An early "Silverface" Bandmaster Reverb head (preferably). A
1968 or 69 are the best platforms for this project, because the later the
model, the more you have to change to blackface & clone it. Second
choice would be a Pro Reverb, which allows you to use the same speaker and
cabinet. Last choice would be a Super Reverb... While a Super Reverb
"can" be used, they're really good amps in their own right, so
they should only be chosen if they're already basket cases or as a last
resort.

A heavy duty grounded 3 conductor power cord if the amp you're starting
with doesn't already have one. Black if you prefer, gray if you want the
original color used on the Vibroverb. (This is optional, but HIGHLY
recommended for your own safety).

For all but the "bare bones budget" model, you'll want to
recap the amp which will require the following:

Poly Tone Caps (Either Mallory 150 or Sprague Orange Drops).

6 - .1

3 - .047

1 - .02

1 - .003

Ceramic Disc Caps (Buy high tolerance, not Radio shack brands).

1 - .001

2 - .01

1 - .02

*Don't replace smaller one's unless visibly damaged.

Foil Coupling Caps (Sprague Atoms are the best).

7 - 25uf-25v

Foil Bias Supply Cap (Sprague Atoms are the best).

1 - 25uf-50v

Foil Power Supply "Doghouse" Caps (Sprague Atoms are the
best).

3 - 20uf-500v (Next largest available size won't fit).

2 - 70uf-350v (Some people use 450v's).

Resistors for blackfacing and cloning. Note, you should also
measure out and replace any existing resistors that have drifted
more than 10-15% beyond their original value. Note, resistors that
are soldered into the card can't be measured accurately, they need to be
de-soldered to get an accurate measurement. Note, the original
Vibroverbs used Carbon Composition resistors. Newer metal film
resistors can be purchased more cheaply, but they have a someone clearer
tone that isn't as warm as Carbon Comp.

Bias Circuit Resistors

1 - 100k ohm - .5 watt

1 - 82 ohm - .5 watt

2 - 220k ohm - .5 watt

1 - 27k ohm - .5 watt

1 - 470 ohm - "1" watt

Negative Feed Back Loop Resistor

1 - 47 ohm - .5 watt

Bass Potentiometer Resistor

1 - 6800 ohm - .5 watt

*(Only necessary if removing the Trem channel Mid pot).

5. "3" Pin, 10K, Linear, Bias Potentiometer. (This is
optional and only necessary for complete historical accuracy). The
Silverface "4" pin bias balance pot can be wired for blackface
use. The only source I'm aware of for "3" pin bias pots is
Fender, they use them in their reissue amps.

6. 15" Speaker. You can use either a vintage JBL D130F Alnico, a
Jensen C15N ceramic, or a CTS 15" ceramic if you want to make the amp
historically accurate. Otherwise, any 15" guitar speaker capable of
handling at least 40 watts will do. I personally like the JBL best because
of its alnico magnet that causes the speaker to have a smoother breakup
and the natural addition of "compression" at stage volumes. The
ceramics are better for a cleaner tone with more headroom. Additionally,
if the speaker you use is already blown, Ted Weber, owner of Webervstdoes an excellent job of reconing and restoring them back to original
condition.

7. Cloth Wire for redressing the amp's wiring back to blackface
specification. (THIS IS COMPLETELY OPTIONAL). I did this on my amp,
which you will see pictures of the before and after later on in this FAQ.
This takes a LOT of time, patience, and attention to detail to do it
correctly. I did it just to see if I could recreate the same lead dress
that Leo Fender did from the Factory back in 1964. It's also a great way
to quiet a noisy amp by dressing the leads the way they were intended by
the original designer. But it's also a great way to screw up! If you mis-wire
just ONE wire, you could end up chasing your tail for a loooooong time to
find your mistake. This process is definitely NOT RECOMMENDED for the
faint of heart. But if you're as stubborn as I was, here are the
lengths and colors of the wire necessary to rewire a complete AB763
Vibroverb. Note these numbers don't allow for much waste, so if you're not
being very careful with measurements, you might want to pad the quantities
by a couple extra few each.

#18 Black - 8'

#20 Brown - 8'

#20 Red - 8'

#20 White - 5'

#20 Blue - 10'

#18 Green - 10'

#20 Yellow - 16

8. Output Transformer (Optional). You can use the original OT if you're
using a Bandmaster Reverb or Pro Reverb chassis. But if you're using a
Super Reverb you'll have to get an 8 ohm replacement. If you want to build
the SRV model, you'll need a Twin Reverb OT. If you want to build a 99.9%
Vibro-Clone, you'll need an original spec paperwound Vibroverb OT, which
can be bought from Larry Rogers of Roger's
Amps in South Carolina.

9. Faceplate (Optional). For this, you need to decide if you want to
keep your mid control on the Tremolo channel or not. If you're
"keeping" it, you "can't" use a real Vibroverb
reproduction plate. You'll have to either keep the original silverface
plate, or you can use a reissue blackface Super Reverb faceplate, or you
can get one of the "Vibro-Clone" faceplates that are currently
being made by a few different sources.

10. Cabinet. You can get an inexpensive cabinet from many suppliers.
You can get one with most of the hardware like tilt back legs and a logo
for a little more. Or if you want a premium cabinet that's original down
to the very last detail, you can do like I did and have Larry Rogers of Roger's
Amps in South Carolina build it for you. I can personally attest
that he does EXCELLENT work. (See my pics furthur down in the FAQ).

What services will I need from others ?

Reconing

JBL D130F's are particularly hard to have reconed because JBL has
stopped carrying the reconing kits for this particular model. Ted Weber
of Webervstis the only
person I'm aware of that has the proper materials to do this right. He's
actually had his private run of vintage spec materials commissioned just
for this purpose.

Ceramic Jensen C15N's and CTS 15's are fairly easy to
recone and can be done by just about any competent reconing shop including
Ted Weber.

My personal favorite for the great level of detail as well as turn
around time is Larry Rogers of Roger's
Amps in South Carolina. But there are several other sources for
quality cabinets including Mojotone
and many others.

Blackfacing

Blackfacing is a fairly basic process that is easy to follow if you get a
copy of the Fender Vibroverb AB763 Layout Diagram
and AB763 Vibroverb Schematic. The following
are the changes necessary to accomplish the process:

Note: As with all electronic changes you make, before you ever apply
power to the circuit, double, triple, and quadruple check your changes with
the Fender Layout Diagram using both your
eyes for visual confirmation and your Digital Volt Meter for absolute
electronic component value confirmation. Also pay attention to capacitor
positive and negative orientation where applicable.

Blackface Lead Dress

This is where we separate the men from the boys, or should I say the
historically-accurate-anal-retentive-screw-counter-perfectionists (of which I
am a proud member), from those which are perfectly happy with a functionally
working facsimile of a real blackface amp.

Does going through the hard work and sweat of replacing all the wiring in
your amp with cloth covered period correct wiring and making sure the lead
dress twists meet a perfection standard beyond even what Leo's own assemblers
ever created make the amp better tone-wise? I can't really say... But what I
can say is that after taking on this monumental task, my amp is pretty much
dead quiet and blackface-like in tone. And the act of taking on such a labor
of love definitely "makes the amp your own". What did your mother
always tell you...? "Anything worth having isn't easy...".

Well, this wasn't easy, but I'm definitely glad that I've had the
experience of walking in the footsteps of those long gone assemblers from Leo
Fender's illustrious Blackface Era of amplifiers.

BEFORE < - > AFTER

(Click each Image to See Enlarged Chassis View)

Blackface Lead Dress

Use the Layout Diagram to determine the
proper color wire for each location. Use #20 or #22 gauge wire in all
locations except for the ground wire coming from the doghouse and the
heater wires. For those, use #18 gauge.

Make your twists "numerous" and "tight" in any
location on the Layout Diagram where you
see wires "grouped". This includes wiring coming off of the pots
and the bundle that comes from the doghouse along with the wires that come
from the bias pot.

Drill 1/8" holes in each location shown on the Layout
Diagram, on the "upper" circuit card (not all the way
through both). Dressing the wires through these holes will greatly reduce
the "cluttered" look of your wiring as well as reducing noise in
the amp from crosstalk of adjacent wires.

Blackface Grounding Scheme

Blackface Ground changes

The grounding scheme of silverface amps changed primarily in two
locations, the first being that of the power transformer and doghouse
70uf-350v ground tap near the bias pot. Silverface amps combined the two
at one ground pad, while the blackface era amps separated the two by about
1 1/2" giving them both their own grounding pad on the chassis.

The second change was in the doghouse itself off of the 20uf-500v
capacitors. Instead of grounding each 20uf with it's own ground tap, the
blackface era amps chained the negative ends of the capacitors together
with heavy ground wire and had one single ground tap coming off of the
20uf on the end of the circuit card that was attached to the brass
grounding strip near the normal channel's treble pot.

Negative Feedback Loop Circuit Changes

Heater Wiring Circuit Changes

Heater Wiring

At the pilot lamp, you may find two 100 ohm resistors to ground. And
looking at the AB763 diagram, you'll notice they aren't there. You
SHOULD NOT remove these. Their purpose is for a ground reference on
the secondary heater tap of your Power Transformer. Older power
transformers had a built in ground center tap, while newer power
transformers did not. If you remove these, you're likely to induce a
great degree of 60 cycle hum, so don't.

Tremolo Circuit Changes

Recapping

When recapping your amp you have lots of choices, way more than I'm
prepared to discuss in this section. But Sprague Atom caps are commonly
considered the best for the Power Supply filter caps in the
"Doghouse" and the Cathode Bypass caps used
"inside" the chassis (the 25-25 caps).

Ceramic caps normally don't need to be replaced unless they become
microphonic (tapping on them with a non metallic object while the amp is on
will cause sound to come through the speaker), or if the capacitor looks
cracked near the lead entry points. If you do replace them, make sure to use
high grade close tolerance ceramics, not the cheaper types found in places
like radio shack.

For the tone caps "inside" your amp (the non-ceramic caps that
aren't the Cathode Bypass caps mentioned above), you have two choices which
can greatly affect the tone of your amp.

Polypropylene - These caps yield a very clear warm almost
audiophile tone and typically come in an axial lead configuration. (Leads
exit the bottom of the cap causing the cap to stand up off of the board).

Sprague Orange Drops are "Polypropylene" rated at 600v

Polyester - These caps yield a slightly darker vintage
"brown sound" tone and typically come with a radial lead
configuration. (Leads exit the ends of the cap and allow the cap to lay
flush on the circuit card).

The Doghouse

The Doghouse

The resistors in the doghouse should match the Fender
Schematic 220k, 220k, 1k, and 4.7k.

The capacitors in the doghouse are the hardest working parts in your
amp and if/when they fail, they usually create a dramatic fireworks
show. So it's typically a good idea to replace them. The values for
these caps are three 20uf-500v and two 70uf-350v

Vibro-Cloning

Design Changes

Vibroverb design changes

Removal of the mid pot (which is optional), requires the shifting down
and rewiring of the volume, treble, and bass pots. Additionally, the bass
pot gets a 6800 ohm resistor that the original didn't have to compensate
for the tonal changes caused by the removal of the mid control.

Change the 100k Negative Feedback Loop voltage divider resistor to a 47
ohm. This is the one which comes after the 820 ohm resistor connected to
the Ext Speaker lead (the only resistor on the board mounted
horizontally).

The Output Transformer used in a Vibroverb was typically a paper wound 8
ohm load model, this one will yield more clean head room and more bass
response than a Bandmaster Reverb OT.

Optionally you can use an original Bandmaster Reverb OT, but your
clean headroom will be lower and the amp will distort sooner.

If you're using a Pro Reverb, the existing stock transformer should
be sufficent.

If you're using a Super Reverb, you'll have to change out the OT
because a SR transformer only expects to see a 2 ohm load. Your only
other option is to have your speaker reconed with a 2 ohm voice coil
load.

If you're building an SRV Vibro-Clone, then you'll want to use a
Twin Reverb output transformer. This will increase the clean headroom
even more over the original Vibroverb OT as well as giving the amp an
even greater potential bass response because of the added iron mass.

Grounded Power Cord

There's NO REASON not to change your amp over to a 3 conductor grounded
power cord if it's currently only got a 2 conductor non-grounded cord. This
doesn't affect the value of your amp and in the end could be the difference
between a well operating amp and one that could electrocute you.

There's also NO REASON to keep the ground switch in this circuit after
you've installed a grounded power cord. Many people do this not realizing that
if the cap connected to this switch ever fails, you'll end up with 120vac on
your guitar strings. The original switch was meant to switch the hot and
neutral leads in the case of a miswired outlet to reduce hum. The grounded
lead of your new cord makes this unnecessary. The amp isn't going to hum any
more or less no matter what the orientation of the hot and neutral wire are
now that the chassis has a "true" ground.

Additionally, if you're amp came with a 2 conductor power cord, it probably
also came with a 2 conductor utility socket. Many people don't use these, but
I find them to be a great way to turn my effects power supply on and off. So
if you want, you can change your 2 conductor to a grounded socket now. The
original Amphenol part can still be found in places like Vibroworld.

Bias'ing Power Tubes

Biasing can be done in a number of different ways, many of which are valid.
But I've found that my favorite way is simple, effective, very accurate.

Before you ever turn the amp on, set your bias pot to the middle of it's
range, turn all your control pots to '0', and put the standby switch in
the 'ON' position. (This would be leaving the switch in the same position
as the power switch 'OFF' position). Then connect a "dummy load"
to your Speaker Jack. I personally just use an old speaker that a wrap in
a towel to muffle any noise. But you can also use an 8 ohm 200+ watt
resistor connected to a 1/4" phono jack too. (Just don't use your
brand new 15" speaker for this purpose, noises that can be created by
an unstable amp could damage the speaker).

Set your DVM for Negative DC voltage measurement and connect the
positive lead to the center pin of your bias pot (the one that has a lead
going to the 220k resistor pair on the circuit card). Connect the negative
lead to one of the power transformer screws. Then power on the amp and try
to adjust the bias pot to get a reading of -44vdc to -50vdc. Now you've
got the amp bias "ballparked". Note: if you can't adjust
the amp to this range, you either have a component in the amp that's
drifted too far out of spec or a wiring/component mistake. You may be able
to change the 15k resistor on the bias pot to a different value to
compensate for the difference, but you really should find out what is
causing the problem instead of using the bias pot resistor to fix the
symptom instead of the problem.

Now, power down, put the positive lead on pin #3 of one of your power
tubes while leaving the negative lead on the power transformer screw for a
ground. Turn the amp on again and notate the DC voltage reading. Then
power down, move the positive lead to pin #3 of the other power tube and
repeat the process. You now have your Plate Voltages. Note:
Make sure these two readings are either identical or at least very close.
If they aren't, then again, you may have a component drift problem or a
component/wiring mistake.

Now, go to the Download section of The Vibro-Clone Workshop and
download the "Plate
Load Calculator" from Duncan
Amp's. Launch the program (you'll need to have Microsoft Excel),
and put the plate voltage you notated in the previous step into the
spreadsheet where it calls for it. See the next step for the
"dissipation rate" to complete the necessary information to run
the calculator.

The two tubes that you have a choice of in the Vibro-Clone are the
original spec 6L6GC or for a little darker tone and slightly earlier
breakup you can use my favorite, the 5881/6L6WGB. The dissipation rates
for each are as follows.

27v @ 70% - 6L6GC

23v @ 70% - 5881/6L6WGB

The resulting value you see listed in the green to yellow area on the
bar in the calculator, represents the nominal value for setting your bias.
Note: If you don't have Microsoft Excel and you can't run this
calculator. An average bias value that will work good for either tube in
most cases is -35ma. The calculator just enables you to adjust it more
precisely based on your specific plate voltages.

Now go back to your powered down amp, set your Digital Volt Meter
for -ma (milliamps). This will probably require you to move the probe
leads on the meter to different plugins as well. Then put your positive
lead on pin #3 of one of the power tubes and put the negative lead on the
red center tap wire from your Output transformer. The center tap can be
most easily attached to from the small wire connecting the red
center tap and a black lead on the lower left corner of the circuit card
as seen on the Layout Diagram. Now power
up the amp in standby mode with all the pots still turned down to '0' and
adjust your bias pot to the value you got earlier from the Plate
Load Calculator. Power down the amp, move the positive lead over to
the other power tube's pin #3, power back up, and verify that the reading
is the same. Note: if the readings don't match, the most likely
cause is that you're not using "matched power tubes". But if
you've verified that your tubes "are" matched from a reliable
source, then your problem is most likely related to component drift and/or
a component/wiring mistake.

Testing

If everything went as described in all the earlier steps, you now have your
amp adjusted to the correct bias voltage, and you're not getting any strange
noises, try plugging in a speaker if you're not already using one for the
dummy load. Then verify the following:

Power up the amp with a speaker plugged in but no guitar plugged in yet.
Turn the standby switch 'OFF' and see if it's still quiet.

Start to dial up all the control pots slowly and see if the amp
continues to stay stable until you get them all up to '10'. At this point
you should hear some low volume hiss or hum, this is normal. Any LOUD
noises, squealing, or motor boat sounds are NOT normal.

Now if that all worked, put the amp in standby, plug in your guitar,
switch the standby back 'OFF', and give your amp a whirl to see how it
sounds. If everything sounds good, you're now ready to drop it into your
cabinet.

Note: If you're getting any strange noises, don't despair. I didn't
get mine up the very first time either. This just means you'll need to check
out the troubleshooting FAQ here and possibly ask a few questions over on
"The Crossroads". Most
times, you'll find that the problem is a simple wiring or component value
mistake.

Final Assembly

Now that you've got your amp working properly, it's time to drop it into
it's new home. If you're doing a "bare bones budget" or
"player" version, you simply need to put the chassis back into the
head cabinet. If you've purchased a new Vibroverb cabinet, then you'll want to
drop it in there. See the following tips before you start.

Speaker Orientation

If you're using a JBL, the speaker wire connectors will typically be
set at about the 5 o'clock orientation mark. (Other speakers may vary)
Put the speaker on the mounting screws, but don't bolt it in yet. Then
slide in the chassis to make sure the output transformer clears the
speaker frame. If not, rotate the speaker as necessary to accomplish
this. Then bolt it all up.

If you used a larger than normal output transformer for the SRV
Vibro-Clone, then hopefully you remembered to either order the cabinet
with an offset speaker hole or a non-standard mounting screw
orientation to accommodate your speaker.

Faceplate

If you're using the original faceplate that came with your
particular chassis, you can skip this step. If you're using a hybrid
"Vibro-Clone" or reissue Super Reverb plate to accommodate
keeping the mid control, then remove the old plate carefully. If
you're using an original Vibroverb faceplate, make sure you've
remembered to remove the mid control and shift all your adjacent pots
down one hole. The last hole will be covered by the faceplate now. Note:
Remember that in addition to removing all the switch screws, knobs,
pot nuts, and the pilot lamp, you also need to remove the inner sleeve
of the pilot lamp which is held on by a nut inside the chassis.
Failure to do so will result in damage to your old faceplate and an
inability to install the new one. Note: If you're using a
reissue Super Reverb faceplate, you'll need to slightly enlarge the
pot holes to accommodate the silverface era sized pots.

New Cabinet Chassis Holes

If you've already tried bolting up your chassis before reading this,
about now you're probably noticing that the rear holes in the chassis
don't match up. This is because the head mounted chassis like the
Bandmaster Reverb use a more narrow chassis strap. Simply take your
new chassis strap, line it up with the hole nearest the faceplate,
mark the location of the rear hole, and carefully drill a new one with
a 11/64 drill bit. (MAKE SURE YOU DON'T HIT ANY INTERNAL WIRING OR
COMPONENTS). Now you're ready to bolt it up!

Reverb tank leads

Make sure you install your reverb tank so that the leads dress up
the right side (as looking into the amp from the back). Otherwise,
you'll induce an excessive amount of interference hum into the reverb
circuit from the power transformer.

Troubleshooting FAQ

If you've ended up here don't despair, my amp didn't work the first time I
powered it up either. Mine turned out to be a wrong value component that I had
installed which was supposed to be in ohms, but I the component I used was the
same value only in 'K' ohms. It happens...

Hopefully you tested out the amp BEFORE you started your modding. This will
save you a LOT of heartache if you verified that everything was working before
you started. Otherwise you end up needing to troubleshoot a LOT more unknown
factors unnecessarily.

If your amp isn't working at this point, go to the Troubleshooting FAQ
section of The Vibro-Clone Workshop. There you'll find a checklist to go
through. If that doesn't help, you can go to the Discussion section
which will point you to "The
Crossroads" discussion forum for online live help.

CAUTION !!!

Amplifiers contain voltages high enough to
KILL you even after the amp has been powered Off !!! If you don't have enough electronic
experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. Ifyou
still want to proceed with learning how to work on amps, I along with many
others will give you advice. But any I accept no responsibility or liability for the outcome of using any
of the information listed in this website. The information
contained within is provided for informational purposes only and is to be used
AT YOUR OWN RISK!

The Vibro-Clone Workshop is a privately owned website and is in no way
affiliated with Fender Musical Instruments Corporation (“FMIC”).
Fender Tm, Vibroverb Tm, Bandmaster Reverb Tm, Super Reverb Tm, and Pro Reverb
Tm are all trademarks of [FMIC]. All " other" products mentioned are registered
trademarks or trademarks of their respective companies.